Reaction of sodium carbonate with sulfur dioxide to form sodium metabisulfite (Na.sub.2 S.sub.2 O.sub.5) proceeds in accordance with the following equations: EQU (1) Na.sub.2 CO.sub.3 + SO.sub.2 .fwdarw.Na.sub.2 SO.sub.3 + CO.sub.2 EQU (2) Na.sub.2 SO.sub.3 + SO.sub.2 .fwdarw.Na.sub.2 S.sub.2 O.sub.5
These reactions are well known, yet their commercial application has posed many difficulties. In one known process sodium carbonate is suspended in a concentrated solution of sodium sulfite and the suspension is passed serially through cascading absorber vessels countercurrent to a sulfur dioxide-containing gas stream. The suspension in the vessel into which the sodium carbonate is introduced, and through which the gas stream passes last, is maintained on the alkaline side to minimize sulfur dioxide emission in the exit gases. There is obtained in the absorption vessel through which the sulfur dioxide-containing gas stream passes first an acidic suspension of sodium metabisulfite crystals. The pH of the liquid in the intermediate vessels ranges from alkaline through neutral to strongly acidic. This process has two outstanding disadvantages.
First, the intermediate vessels of necessity contain very heavy slurries of sodium sulfite. Under the process conditions, approximate solubilities of sodium metabisulfite, sodium sulfite and sodium carbonate are about 40 percent, 23 percent and 30 percent, respectively. In order to obtain acceptable space-time yields, sodium metabisulfite concentration in the vessel through which the sulfur-dioxiding containing gas stream passes first must be maintained well above 40 percent, on the basis of the slurry. Hence, it is readily apparent that the intermediate vessels will be heavily loaded with relatively less soluble sodium sulfite crystals. In this type of process, the intermediate vessels and connecting pipes, especially the valves in the connecting pipes, tend to become choked by enrusted crystalline sodium sulfite. Further, these sodium sulfite crystals are abrasive, causing rapid wear of valves and pumps.
Second, sodium sulfite oxidizes with comparative ease to form sodium sulfate, especially under slightly alkaline or neutral conditions. As is readily apparent, the intermediate vessels containing large amounts of suspended sodium sulfite must pass through weakly alkaline and neutral stages, so that undesirable sodium sulfate is formed which gradually builds up in the process liquors and can be removed therefrom only by purging the liquors. In order to meet sulfate specifications for the metabisulfite product, sulfate contamination may not exceed certain maximum limits, which can be maintained only by purging liquors from the process, resulting in product losses, unless other uses can be found for the purge liquor.
To overcome difficulties caused by sulfate formation, Melendy in U.S. Pat. No. 2,245,697 proposed to conduct the reaction of sodium carbonate and sulfur dioxide to form sodium metabisulfite in a series of two absorption vessels wherein the process liquors in each of these absorption vessels is maintained under acidic conditions. To that end, Melendy slurries sodium carbonate in sodium metabisulfite process mother liquor in a separate mix tank to obtain a strongly alkaline emulsion, which emulsion be then feeds into the second of the two absorption vessels. In that second absorption vessel the sodium carbonate react with sulfur dioxide to form sodium sulfite crystals. The slurry in the second absorption vessel is maintained under slightly acidic conditions. Slurry from the second absorption vessel is passed to the first absorption vessel wherein the sodium sulfite is reacted with sulfur dioxide to form sodium metabisulfite. The slurry in the first vessel is maintained under strongly acidic conditions. Sulfur dioxide is introduced directly into both the first and the second absorption vessel. Off-gases from the first absorption vessel are passed to the second absorption vessel, and off-gases from the second absorption vessel are exhausted to the atmosphere. The Melendy process substantially reduces formation of sodium sulfate byproduct because it avoids need for maintaining sodium sulfite under slightly alkaline or neutral conditions. The Melendy process, however, does not avoid need for handling highly concentrated slurries of sodium sulfite, with concomitant difficulties of equipment wear and crystal encrustation.
Difficulties arising from need for handling heavy sodium sulfite slurries is avoided by the process disclosed in U.S. Pat. No. 3,860,695 to Metzger et al. In that process formation of crystalline sodium metabisulfite is effect by feeding sulfur dioxide-containing gases into weakly acidic sodium metabisulfite mother liquor in a jet scrubber submerged in the mother liquor. Sodium metabisulfite product crystals are separated from the mother liquor, the mother liquor is pump-circulated, and sodium hydroxide or sodium carbonate solution are added to maintain the pH of the mother liquor at from 4 to 5. This process, unfortunately, has the disadvantage that the residual off-gas leaving the reaction vessel contains substantial amounts of unreacted sulfur dioxide, so that it cannot be exhausted directly to the atmosphere. Hence, the patentees recommend that it be used to make sodium hydrogen sulfite or sodium sulfite in a separate apparatus or plant. Since this is not always economically feasible, the Metzger et al. process has severe limitations.
It is an object of the present invention to provide an improvement in the process for making sodium metabisulfite avoiding disadvantages of prior art processes.